The field of the invention is electron paramagnetic resonance (EPR) spectroscopy and, more particularly, the automatic frequency control of the spectrometer and acquisition of the EPR signals.
EPR measurements are usually performed utilizing a reference-arm microwave bridge such as that shown in U.S. Pat. No. 4,593,248. In such a system, a microwave source oscillator is tuned to the resonant frequency of a microwave resonator. The source oscillator output is divided onto two different paths; a reference arm and a signal arm. Both the reference arm and the signal arm include attenuators to adjust the signal strength in their respective paths.
The signal arm includes a circulator for coupling to a resonator containing the sample under study. The reference arm functions as a local oscillator and produces an output signal which is mixed with the circulator output. The resulting signal from the mixing process is then detected. In the detection process, the phase of the signal in the reference arm is critical, and must be adjusted to optimize the detected signal. For that purpose, prior bridges required a variable phase shifter in the reference arm.
Either one of two orthogonal EPR signals, referred to in the art as "Absorption" and "Dispersion", can be detected utilizing a single channel detector by suitable adjustment of the phase shifter in the reference arm. The oscillator is first tuned to the resonant frequency of the resonator. The sample in the resonator is then subjected to a strong d.c. "polarizing" magnetic field and at the same time irradiated by a radio frequency "source" magnetic field at the electron's frequency of precession.
At that point, gyromagnetic resonance occurs, changing the resonator quality factor, or "Q", and shifting the resonant frequency of the resonator. The absorption signal is associated with the change in resonator Q and is in-phase with the incident microwave signal. Because of the "in-phase" condition, the absorption mode signal is also associated with the "Real", or "Re" part of the complex reflection coefficient of the resonator. The dispersion signal is associated with the shift in resonant frequency, and is in-quadrature with the incident microwave signal. Analogously, the dispersion signal is also associated with the "Imaginary", or "Im" part of the complex reflection coefficient of the resonator. Appropriate setting of the phase shifter is thereby essential to detect pure absorption or pure dispersion signals, each mode requiring a phase setting 90.degree. away from the other mode.
Proper phase adjustment in the reference arm is also critical to the operation of Automatic Frequency Control (AFC) circuits in prior reference arm bridges. The microwave source oscillator is frequency modulated at a rate below the source excitation frequency. The latter source excitation frequency as is well known in the art, is the frequency at which a magnetic field modulation is applied to the resonator, and consequently is also the frequency at which the EPR signals are received, usually about 100 kilohertz (kHz). The rate of frequency modulation of the microwave source is termed the AFC frequency, and is typically from 10-70 kHz. The modulation index of the AFC modulation is very low, so that the deviation of the microwave source oscillator from the desired resonant frequency is small.
Prior AFC circuits are known in the art, and operate by discriminating on the Real part (absorption mode) of the microwave energy reflected from the resonator at the AFC frequency, as explained in detail below. It was therefore necessary for the phase of the reference arm to be precisely set so that the detector used for the AFC signal is operated in the absorption mode. If the phase shifter were not optimally set, the AFC circuit performance would be degraded. And if the phase of the reference arm were far enough from optimum, AFC lock could be lost altogether.
The need for phase adjustment in the reference arm of prior bridges is a serious disadvantage. If the bridge is implemented utilizing waveguides, then rotary vane attenuators that have minimal phase shift can be employed. In that case, the phase is fairly stable, but still must be adjusted to ensure purely absorption or dispersion signals and, in some cases, to change between absorption/dispersion detection. If the bridge is implemented utilizing coaxial cable, the phase control problem is even worse. The attenuators currently known for use in coax have an accompanying attenuation dependent phase shift, so that each change of attenuator setting demands a change in reference-arm phase.
In addition to the AFC and phase shift problems enumerated above, prior reference arm bridges suffer from an inability to efficiently detect both absorption and dispersion signals simultaneously. To do so in prior bridges would require splitting the signal to be detected into two branches, each branch with half the signal power. Since separate detection in each branch introduces approximately the same amount of noise, each separate detection suffers a reduction in the signal to noise ratio (S/N) by a factor of .sqroot.2.